Fiber structures including catalysts and methods associated with the same

ABSTRACT

Fiber structures that include a catalytic material are provided. The fiber structures (e.g., membranes) may be formed of interconnected carbon fibers. The catalytic material may be in the form of nanosize particles supported on the fibers. In one method of the invention, the structures are produced by electrospinning a polymeric material fiber structure that is subsequently converted to a carbon fiber structure in a heat treatment step which also causes the catalytic material particles to nucleate on the carbon fibers and grow to a desired size. The catalytic material may be uniformly distributed across the carbon fiber structure and the amount of catalytic material may be controlled. These factors may enhance catalytic performance and/or enable using less catalytic material for equivalent catalytic performance which can lead to cost savings, amongst other advantages. The fiber structures may be used in a variety of applications including electrodes in batteries and fuel cells.

FIELD OF THE INVENTION

The invention relates generally to fiber structures and, moreparticularly, to carbon fiber structures that include a catalyst andmethods associated with the same.

BACKGROUND OF THE INVENTION

A catalyst is a substance that increases the rate of a chemical reactionwithout, itself, being consumed in the reaction. Catalysts function bylowering the activation energy associated with the rate-determining stepin a chemical reaction. As a result, the chemical reaction isaccelerated.

Catalysts may be used in numerous applications. For example,electrochemical devices (e.g., fuel cells, batteries) may utilizecatalysts. These devices can include electrodes (i.e., an anode and acathode) with a solid or liquid ionic conducting and electronicallyinsulating phase therebetween. Fuel materials are brought in contactwith the anode and an oxidizing gas (e.g., oxygen) is brought in contactwith the cathode. The fuel material may be oxidized in a chemicalreaction which may be accelerated by the presence of the catalyst at theanode. The oxidizing gas is reduced in a chemical reaction which alsomay be accelerated by a catalyst at the cathode. The device generateselectricity when electrons generated in the fuel oxidation reaction atthe anode flow through an external circuit to the cathode where theelectrons are consumed in the reduction reaction.

A number of different materials may be suitable catalysts for reactionsthat occur at electrodes in electrochemical devices. Examples ofsuitable catalysts include metals such as platinum and palladium, aswell as alloys and compounds thereof. Because such catalytic materialsare relatively expensive, it would generally be desirable to use lesscatalytic material if sufficient catalytic activity can be maintained.

SUMMARY OF THE INVENTION

The invention provides fiber structures (e.g., membranes) which includecatalytic material, as well as methods associated with the same. Thestructures may be used, for example, as electrodes in electrochemicaldevices.

One aspect of the invention involves methods of making structures. Inone embodiment, a method of forming a carbon fiber structure isprovided. The method comprises electrospinning a polymer solution toform a polymeric material fiber structure. The method further comprisesheat treating the polymeric material fiber structure to convert thepolymeric material fiber structure to a carbon fiber structure includingcatalytic material particles supported on the carbon fiber structure.

In another embodiment, a method of forming a carbon fiber structure isprovided. The method comprises converting a polymeric material fiberstructure to a carbon fiber structure, and associating a catalyticmaterial with the carbon fiber structure.

In another aspect, a structure is provided. The structure comprises acarbon fiber structure including at least one carbon fiber having alength of greater than about 500 microns. A catalytic material issupported on the carbon fiber structure.

In another aspect, an electrode is provided. The electrode comprises acarbon fiber structure and a catalytic material supported on the carbonfiber structure. The catalytic material concentration is less than 0.2mg catalytic material/cm² area of the carbon fiber structure.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. Theaccompanying figures are schematic and are not intended to be drawn toscale. In the figures, each identical, or substantially similarcomponent that is illustrated in various figures is represented by asingle numeral or notation. For purposes of clarity, not every componentis labeled in every figure. Nor is every component of each embodiment ofthe invention shown where illustration is not necessary to allow thoseof ordinary skill in the art to understand the invention. All patentapplications and patents incorporated herein by reference areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of a carbon fiber structure supportingcatalytic material particles according to one embodiment of theinvention.

FIG. 2A illustrates a proton exchange membrane fuel cell includingcarbon fiber structures as electrodes according to an embodiment of theinvention.

FIG. 2B shows the electrode assembly used in the device of FIG. 2A.

FIGS. 3A-3C are respective copies of SEM images at differentmagnifications of a carbon fiber structure of the invention as describedin Example 1.

DETAILED DESCRIPTION

The invention provides fiber structures that include catalytic material.The fiber structures (e.g., membranes) may be formed of interconnectedcarbon fibers. The catalytic material may be in the form of particlessupported on the fibers. In one method of the invention (i.e., theelectrospinning/heat treatment method), the structures are produced byelectrospinning a polymeric material fiber structure in the presence ofa catalytic material precursor. The polymeric material is subsequentlyconverted to a carbon fiber structure in a heat treatment step which mayalso cause the catalytic material particles to nucleate on the carbonfibers and grow to a desired size. The catalytic material may beuniformly distributed across the carbon fiber structure and the amountof catalytic material may be controlled. These factors may enhancecatalytic performance and/or enable using less catalytic material forequivalent catalytic performance which can lead to cost savings, amongstother advantages. The fiber structures may be used in a variety ofapplications including as electrodes in batteries and fuel cells.

FIG. 1 shows a fiber structure 10 according to one embodiment of theinvention. The structure is formed of interconnected carbon fibers 12.Catalytic particles 14 are supported on the carbon fibers. During use,the particles catalyze desired chemical reactions between reactants thatare exposed to the particles. As described further below, the particlesare distributed throughout the fiber structure which can lead to uniformcatalytic activity across the structure which may result in performanceadvantages.

Although the catalytic material is in the form of particles supported oncarbon fibers in the illustrative embodiment, the catalytic material maybe in other forms (e.g., coatings) and/or otherwise associated withfiber structures in other embodiments.

In many embodiments and applications, it may be preferable for thecarbon fiber structure to be a membrane. Membranes generally arerelatively thin sheet-like structures. However, non-membrane structuresare also encompassed by the invention. For example, the fiber structuremay be a bundle or spherical, amongst others.

Structure 10 may have any suitable dimensions. The dimensions may bedictated, in part, by the application in which the structure is used. Insome embodiments (e.g., when the structure is a membrane), the structurehas a dimensional area on the order of 1 cm² to 500 cm². However, itshould be understood that the structures may also have a wide range ofareas outside this range.

The structure may be formed over a range of thicknesses. Typicalthicknesses are on the order of microns (e.g., when the structure is amembrane), though other thicknesses are also possible. In someembodiments, the structure is very thin. It may be particularlyadvantageous to use thin structures to form electrodes, as describedfurther below. For example, the structure (and electrode) thickness maybe less than 10 microns; in some cases, less than 5 microns; and, insome cases, even less than 1 micron. In some cases, it may be beneficialfor the structure (and electrode) thickness to be greater than 0.1micron to ensure sufficient mechanical integrity and catalytic activity.In particular, the electrospinning/heat treatment method of theinvention enables formation of very thin structures.

The illustrative structure includes a plurality of pores 16 formedbetween the carbon fibers. The pores allow reactants (e.g., gaseousspecies) to pass through the structure, while being exposed to thecatalytic particles which catalyze the desired reactions. The poresgenerally do not extend across the entire the structure (along any oneaxis). However, many pores may be interconnected to form pathways thatdo extend across the structure (along any one axis) so that thereactants (e.g., gaseous species) may flow through the structure.

The structure may be formed of a plurality of interconnected carbonfibers. However, it is also possible for a single carbon fiber to beintertwined to produce the structure. For example, in someelectrospinning/heat treatment methods, a polymeric material structureis formed from a single fiber in the electrospinning step which issubsequently converted to a carbon fiber structure in the heat treatmentstep. Although, even when using this electrospinning/heat treatmentmethod, the structure is more typically formed of a plurality of carbonfibers because of the tendency of polymeric material (and/or carbon)fibers to break during processing.

In certain embodiments, some of the carbon fibers may be fused togetherat points where the fibers intersect. This may arise when polymericmaterial fibers are fused together during the beginning stage of heattreatment which are later converted to carbon fibers that remain fusedtogether at intersection points. This fusion may lead to increasedmechanical integrity and/or increased conductivity. It should beunderstood that some of the fibers may be merely in physical contactwith one another at intersection points without being fused together.

Carbon fibers 12 comprise at least some amount of carbon and, in mostcases, carbon is the primary component of the fibers. However, thecarbon fibers may also comprise other elements. For example, the carbonfibers may comprise nitrogen and/or hydrogen (or, other elements) thatare residual from the polymeric material formed in theelectrospinning/heat treatment method. In these cases, the conversion tocarbon during heat treatment is not complete. In some embodiments, theweight percentage of carbon in the carbon fibers is greater than about75% (e.g., between about 80% and 95%, or between about 90% and 98%)based on the total weight of the fiber. It should be understood that, insome embodiments, the fibers are formed entirely of carbon.

In general, the carbon fibers should have high enough carbonconcentration to be sufficiently conductive for the application in whichthe structure is used. In some embodiments, the fibers (and structure)have a resistivity of less than about 0.1 Ohms-m. For example, theresistivity may be between about 10⁻² Ohms-m and about 10⁻⁶ Ohms-m; or,between about 10⁻² Ohms-m and about 10⁻⁴ Ohms-m. The specificresistivity (and conductivity) of the fibers (and structure) may dependon the relative percentage of carbon in the fibers which, in turn, maybe related to the processing (e.g., heat treatment) conditions.

It should be understood, however, that fibers (and structures) of theinvention may have resistivities outside the above-noted ranges.

Fibers 12 can have any suitable dimension. In some embodiments, theaverage fiber diameter is greater than about 10 nm; in some embodiments,greater than about 50 nm; and, in some embodiments, greater than about100 nm. Fiber diameters less than these ranges may cause the structureto have insufficient mechanical integrity. The fiber diameters may beless than about 500 nm. In some cases, the fiber diameters may be lessthan about 300 nm. Fiber diameters greater than these ranges may be moredifficult to process and/or may not be compatible with certainapplications. It should be understood that the fiber diameters may bebetween the above-noted values (e.g., between about 100 nm and 300 nm,between about 100 nm and about 500 nm, etc.). The average fiber diametermay be determined by measuring the diameter of a representative numberof fibers of the structure, for example, using SEM (scanning electronmicroscope) techniques.

The fibers generally have circular-shaped cross-sections, though othercross-sections may also be suitable. In some embodiments, the fibers arepreferably solid (i.e., not hollow). The electrospinning/heat treatmentmethod can produce solid carbon fibers. However, it should be understoodthat in other embodiments, the fibers may be hollow (e.g., tubes).

Structure 10 may include relatively long carbon fibers in someembodiments of the invention. For example, in some embodiments, at leastone carbon fiber of the structure has a length of at least about 500microns; and, in some embodiments, at least one carbon fiber has alength greater than about 1 mm. In some embodiments, it may be possibleto achieve carbon fiber lengths of greater than about 1 cm, or evensignificantly greater. It should be understood that more than one carbonfiber having the above-noted lengths may also be found in the structuresof the invention. Fiber length may be measured, for example, using SEMtechniques.

In particular, the electrospinning/heat treatment method describedfurther below can produce relatively long carbon fibers. Relatively longcarbon fibers may be advantageous in increasing mechanical integrity andconductivity, amongst other advantages. Electrospinning and long fibersmay also lead to an increase in the number of fibers that are fusedtogether which, as described above, can also lead to increasingmechanical integrity and conductivity.

The microstructure of the carbon fibers may be amorphous. In someembodiments, the carbon fibers may be formed, in part, of graphite. Inthese cases, the graphite portions assume the typical graphitecrystalline structure. The fibers may include a graphite component, forexample, when heat treating temperatures are very high (such as, greaterthan about 1300° C.).

The catalytic material particles may be formed of any suitable catalyticmaterial. In some embodiments, the particles are formed of a catalyticmetal. The following metals may function as catalytic materials incertain applications: palladium, platinum, gold, silver, rhodium,rhenium, iron, chromium, cobalt, copper, manganese, tungsten, niobium,titanium, tantalum, lead, indium, cadmium, tin, bismuth and gallium,amongst others, as well as compounds and alloys of these metals. Inparticular, palladium and platinum may be preferred catalysts for use inelectrochemical devices. Other suitable catalytic materials (and metals)are known to those of skill in the art. The type of catalytic materialdepends, in part, on the reaction to be catalyzed and, thus, theapplication in which the structure is used.

In some embodiments, particles 14 have an average particle size of lessthan one micron. It may be preferred that the particles have sizes onthe order of nanometers. For example, the particles may have an averageparticle size of less than about 50 nm; and, in some cases, less thanabout 20 nm (e.g., on the order of 5 nm). Small particle sizes mayadvantageously lead to the relatively uniform distribution of catalyticmaterial throughout the carbon fiber structure, amongst other positiveeffects.

The particles typically (though not always) have an average particlesizes of greater than 0.5 nm, in part, due to processing limitations. Itmay be difficult to consistently form particles having average sizessmaller than 0.5 nm across an entire structure.

The specific particle size may be controlled, as described furtherbelow, through processing conditions such as heat treatment conditions(e.g., temperature and time). The desired average particle size maydepend on several factors including carbon fiber dimensions andcatalytic performance.

Average particle sizes may be determined by averaging the particle sizesof a representative number of particles using, for example, SEMtechniques. As used herein, the average particle size includes sizes ofprimary particles and sizes of particle agglomerates.

It should be understood that particle sizes outside the above ranges maybe used in certain embodiments of the invention.

It may be preferred for the particle size distribution to be relativelynarrow. Narrow particle size distributions promote the uniformdistribution of catalytic material throughout the structure.

As described further below, in some preferred embodiments, the particlesare relatively homogenously distributed throughout the structure. Also,the particle-to-particle distances on a fiber may be relatively similar.As used herein, the term “particle-to-particle distance” refers to thedistance between adjacent particles on a fiber. For example, at least50% of the actual particle-to-particle distances on fibers of thestructure are within ±25% of the average particle-to-particle distanceon fibers of the structure. In some cases, at least 75% of the actualparticle-to-particle distances are within ±25% of the averageparticle-to-particle distance.

The particle-to-particle distance may be between about 50 nm and 1micron, though other distances are also possible. Theparticle-to-particle distance depends, in part, on the catalyticmaterial concentration.

Structure 10 may be formed using any suitable method. As noted above,one preferred method of forming the structure is an electrospinning/heattreatment method.

Electrospinning is a known technique which involves processing a polymersolution. In some methods of the invention, the polymer solution isprepared by dissolving a polymeric material and a catalytic materialprecursor (e.g., a catalytic metal salt) in a suitable solvent. Themixture is mixed to ensure homogeneous distribution of the polymericmaterial and catalytic material precursor in the solvent. For example,an ultrasonic mixing technique may be used. In some cases, this mixingstep occurs at elevated temperatures (e.g., between about 60° C. andabout 80° C.) for a desired time (e.g., about 2 hours) to promotedissolution.

In some methods, it is preferable to dissolve the catalytic materialprecursor (e.g., a catalytic metal salt) in the solvent prior to addingthe polymeric material. This order of addition may prevent the polymericmaterial from cross-linking during the mixing step.

As described further below, in some methods of the invention, thecatalytic material precursor may be solid particles of catalyticmaterial (e.g., metal oxides). In these methods, the solid catalyticmaterial particles may be dispersed in the polymer solution whichincludes the solvent and dissolved polymeric material.

Suitable solvents are known to those of skill in the art and depend, inpart, on the polymeric material and the catalytic material precursor.Suitable solvents include N,N-dimethylformamide (DMF), ethanol,methanol, acetone, water, tetrahydrofuran (THF), methylene chloride (MCor dichloromethane) and combinations thereof. It should be understoodthat other solvents may also be used.

Any suitable polymeric material may be used in the process. In general,the polymeric material should be compatible with electrospinning andconvertible to carbon upon heat treatment. Suitable polymer materialsinclude, but are not limited to, polyacrylonitrile (PAN), polyethylene,terephatalate (PET), PBI, polystyrene, poly(2-hydroxyethylmethacrylate), polyvinylidene fluoride, poly(ether imide),styrene-butadiene-styrene triblock copolymer,poly(ferrocenyldimethylsilane), polyethylene oxide (PEO), Rayon, Teflon,DNA, Esthane® 5720, segmented polyether urethane, elastomericpolyurethane urea copolymers, biopolymers (e.g., poly(lactic acid),tetraethyl benzylammonium chloride (TEBAC), polyvinylpyrolidone,polycaprolactone, polycarbonate, poly(vinyl alcohol) (PVA), celluloseacetate (CA), polyacrylic acid (PAA), polyurethane (PU), andpolycaprolactone (PCL). It should be understood that other polymericmaterials may be used as known to those skilled in the art.

Any suitable catalytic material precursors may be used. In embodimentsin which the catalytic material precursor is dissolved in the solvent,the precursor should be sufficiently soluble in the solvent. Examples ofsuitable catalytic material precursors include salts (organic and/orinorganic) of catalytic metals (when catalytic metals are used).Examples of suitable metal salts include palladium (II) acetate andplatinum (IV) chloride.

In embodiments in which the catalytic material precursor is in the formof solid particles, suitable solid particles include oxides of any ofthe catalytic metals noted above. The solid particles, for example, havesizes on the order of nanometers (e.g., 1 nm-50 nm)

The concentration of the catalytic material precursor (e.g., catalyticmetal salt) added to the solution is selected to produce a structurehaving the desired concentration of catalytic material (per area of thestructure). It may be preferable for substantially all of the catalyticmaterial precursor to be converted into catalytic material particles, asdescribed further below. Such substantial conversion can increase theability to precisely control the catalytic material concentration whichcan lead to cost savings associated with the catalytic material. Toyield catalytic material concentrations on the order of about 0.02-0.2mg catalytic material/cm² area of structure, the concentration of thecatalytic material precursor (e.g., catalytic metal salt) may be on theorder of 0.001 M-0.10 M. In embodiments in which the catalytic materialprecursor is in the form of solid particles, the weight percentage ofthe solid particles added to the solution yields a structure includingan equivalent weight percentage of catalytic material.

The concentration of polymeric material added to the solution isgenerally selected for compatibility with electrospinning. Typicalpolymeric material concentrations may be between about 8% and about 12%by weight of the solution, and between about 8% and about 9%. Suchconcentrations generally result in the solution having a suitableviscosity for electrospinning. It should be understood thatconcentrations outside the above ranges may also be used if compatiblewith electrospinning.

The electrospinning process involves exposing the polymer solution to anelectromagnetic field. In one illustrative method, a voltage is placedbetween a capillary tube and an electrically conducting substrate (e.g.,solid metal plates, metal meshes). For example, the tube may be at avoltage of between about 3 kV and about 50 kV, while the substrate isgrounded. The distance between the tip of the capillary tube and thesubstrate may be between about 1 cm and about 50 cm. The polymersolution may be dispensed relatively slowly (e.g., 0.1 to 10 mL/hour)through the capillary tube in drop form. The solvent evaporates to leavea polymeric material fiber which is electrostatically attracted to thesubstrate when the electrostatic forces exceed the surface tension ofthe polymer solution at the tip. In some processes, a continuouspolymeric material fiber is formed, though the fiber may break andsubsequent fiber(s) may be formed. The structure may be removed from thesubstrate once a desired thickness and area is achieved.

A general description of an electrospinning process is provided, forexample, in “Polymer Nanofibers Assembled by Electrospinning”, Frenotet. al, Current Opinion in Colloid and Interface Science 8(2003), 64-75,which is incorporated herein by reference.

It should be understood that other electrospinning processes may be usedto form the polymeric material fiber structure. Also, othernon-electrospinning processes may be used to form the polymeric materialfiber structure. However, non-electrospinning processes may not producefiber structures having all of the advantages described herein.

In some methods, the polymer fiber structures are allowed to dry priorto heat treatment. Although, in other methods, a separate drying step isnot utilized.

The heat treatment step generally involves heating the polymericmaterial fiber structure to sufficient temperatures to convert thepolymeric material to carbon. In methods in which the catalytic materialprecursor is dissolved in the polymer solution, heat treatment alsonucleates and grows the catalytic material particles to the desiredsize. In methods in which the catalytic material precursor is in theform of solid particles, heat treatment can lead to the particles beingsupported on carbon fiber surfaces (but, typically, does not lead toparticle growth). In general, any heat treatment conditions that producethe desired carbon fiber structure including catalytic materialparticles of the desired size may be utilized.

Typically, during heat treatment, the polymeric material fibers willshrink in dimensions when being converted to carbon. For example, thecarbon fiber average diameter may be less than 50% (e.g., about 60%) theaverage diameter of the polymeric material fiber.

In some case, the heat treatment step involves two stages includingheating to a low temperature range followed by heating to a highertemperature range. The low temperature stage may involve heating totemperatures between about 100° C. and about 500° C. to stabilize thepolymeric material. The heating rate may be, for example, between about0.5° C./minute to 10° C./minute (e.g., 1° C./minute). This stabilizationstep is typically carried out in air and may last between about 10minutes and about 10 hours with the time depending, in part, on thetemperature. In this stabilization step, bonds (e.g., carbon/hydrogenbonds) may be broken to break down the polymeric material structure.

The second stage may involve heating to higher temperatures, such asbetween about 500° C. and about 3500° C., and, more typically betweenabout 800° C. and about 1100° C. The second stage may involve heattreating the material in an inert atmosphere (e.g., argon) at timesbetween about 1 minute and about 120 minutes. The heating time depends,in part, on the temperature with shorter times being used at highertemperatures. In some methods, it may be preferred to use relativelyhigh temperatures (e.g., 1000° C. or higher) and relatively short times(e.g., 5 minutes or less). Even higher temperatures may be used when thecatalytic material precursor is in the form of solid particles (e.g.,metal oxide particles), because growth of such particles generally doesnot occur.

During the high temperature stage, the polymeric material fibers areconverted to carbon (at least to some extent, though other residualelements may remain as described above). In methods in which thecatalytic material precursor is dissolved in the polymer solution, thehigh temperature stage also leads to particle nucleation and growth tothe desired size.

The temperature and time conditions may be selected to produce catalyticmaterial particles of desired size. For example, in one illustrativemethod in which the second stage of the heat treatment step involvesheating to 800° C., the particles may nucleate after about 40 minutesand may grow to about 20-30 nm after about 60 minutes. If the time istoo long, the particles may grow above the desired size (e.g., up to 100nm and larger).

It should be understood that the heat treatment step may involve heatingto a single temperature range or more than two temperature ranges. Inany case, the temperature and time of heat treatment is selected producethe desired carbon fiber structure including catalytic materialparticles of the desired size After heat treatment, the carbon fiberstructure including particles may be further processed, if needed, toform the desired final structure. For example, conventional techniquesmay be used to form electrochemical devices from fiber structures of theinvention that function as electrodes in such devices.

In some methods of the invention, the above-described processing stepsare performed continuously. For example, the electrospinning process maycontinually form a polymeric fiber structure which is continuouslytransferred to a furnace for heat treatment. The structure, for example,may be transferred on a movable belt. The structure is continually movedthrough the furnace and subjected to heat treatment at appropriateconditions to form a suitable carbon fiber structure. The carbon fiberstructure may be removed from the furnace in a continuous fashion andfurther processed, if needed, to form the desired structure.

It should be understood that other processes may be used to form thecarbon fiber structure of the present invention. For example, othermethods include growing carbon fibers in a vapor phase transport method,a template synthesis method using epoxy-based solution as carbonprecursors, deterministically using catalytic DC plasma-enhancedchemical vapor deposition (DC-PECVD) and CVD processes, amongst others.However, it should be understood that such processes may not producefiber structures having all of the advantages described hereinassociated with the electrospinning/heat treatment method.

Structures of the invention may be used in a wide variety of catalyticapplications. For example, the structures may be used as electrodes inbatteries, fuel cells and other electrochemical device applications.

FIG. 2A illustrates a schematic proton exchange membrane fuel cell 30including carbon fiber structures as electrodes according to anembodiment of the invention. An electrode assembly 31 (as shown in FIG.2B) includes an anode 32, a cathode 34 and a proton conducting membrane36 which separates the anode and cathode. The anode and cathode areformed of carbon fiber structures of the invention. The electrodeassembly is positioned between respective gas diffusion layers 38 a, 38b. As shown, the gas diffusion layers are inserted into respectivesub-gaskets 40 a, 40 b. Graphite plates 42 a, 42 b are in contact withthe respective gas diffusion layers. An electrical circuit is formedbetween the two graphite plates.

It should be understood that electrochemical devices including carbonfiber structures of the invention may have a variety of otherconstructions not illustrated herein.

As noted above, the methods of the invention can lead to homogeneousdistribution of catalytic material throughout the structure (andelectrode). In particular, electrospinning/heat treatment methods canlead to the homogenous distribution and, especially, methods thatinvolve dissolving a catalytic material precursor in the polymersolution. The homogeneous distribution results, in part, from the smallcatalytic particle sizes, as well as the uniform distribution of theparticles throughout the structure. In particular, theelectrospinning/heat treatment method (using catalytic materialprecursors dissolved in the polymer solution) can lead to the uniformdistribution and small particles sizes by nucleating a large number ofparticles relatively uniformly across the structure. The uniformdistribution enables structures of the invention to exhibit sufficientlyhigh catalytic activity across the entire area of the structure, even atlow catalytic material concentrations.

Furthermore, methods of the invention can accurately control thecatalytic material concentration. The concentration may be controlled byadding precise amounts of catalytic material precursor to the polymersolution in the electrospinning process. Also, selecting heat treatmentconditions that convert substantially all of the catalytic materialprecursor to catalytic material particles.

As a result of the accurate control and the sufficient catalyticactivity at low concentrations, structures (including electrodes) of theinvention may have low catalytic material concentrations which canresult in significant material cost savings as compared to otherconventional techniques which use higher catalytic materialconcentrations.

In some embodiments, the catalytic material concentration may be lessthan 0.2 mg catalytic material/cm² area of the structure (or electrode)and, in some cases, less than about 0.1 mg/cm (e.g., about 0.05 mg/cm²or even about 0.02 mg/cm².). In certain embodiments, it may be desiredfor the catalytic material concentration to be greater than about 0.02mg/cm² to provide sufficient catalytic performance.

The electrospinning/heat treatment methods of the invention may alsoenable production of very thin structures (including electrodes) withthicknesses noted above. Thinner electrodes may enable using lower totalamounts of catalytic material and improved electrical performance,amongst other advantages.

The following is an example which is provided for illustrative purposes.The example is not limiting in any way.

EXAMPLE 1

This example illustrates production of a carbon fiber structureincluding catalytic particles according to methods of the presentinvention.

A clear PAN polymer solution was prepared by 1) adding 11 wt % PAN into7.7 g (8.2 milliliter) N,N-dimethylformamide (DMF), 2) mixing with amagnetic stir bar at room temperature for 2 hours, and 3) heating at 80°C. for 20 minutes. A 0.05 M salt solution was prepared by addingpalladium (II) acetate in 2.0 milliliter DMF. The PAN solution was addedthe salt solution. The final solution was mixed for 5 minutes with amagnetic stir bar at room temperature for 5 minutes to ensurehomogeneity.

The homogeneous solution was processed using an electrospinning process.The electric field voltage ranged between 10 kV and 18 kV. Thetip-to-plate distance was about 12 cm. The electrospinning processformed a polymeric material fiber membrane.

The polymeric fiber membrane was heat treated in a two stage process.The first stage involved heating to temperature of 280° C. in air for120 minutes at a heating rate of 1° C./min. The second stage involvedheating to 800° C. in argon for various times. The fiber structure wasanalyzed using an SEM technique at different times during the secondstage.

SEM analysis showed that after about 45 minutes, small Pd particlesbecame observable. After about 60 minutes, the particle sizedistribution was roughly homogeneous and particle size was about 20-30nm. After about 75 minutes, some of the particles had become greaterthan about 50 nm.

FIGS. 3A-3C are respective SEM photos (at increasing magnification) thatshow the resulting structure of the carbon fiber membrane after 60minutes of heat treating at 800° C. FIG. 3C shows a homogeneousdistribution of the palladium (i.e., catalytic material) particlessupported on carbon fibers of the membrane. The particles have anaverage particle size of between about 20 and 30 nm.

This example shows how methods of the invention may be used to produce acarbon fiber membrane which includes catalytic material particlessupported on the carbon fibers. This example also shows the ability tocontrol catalytic material particle size by processing conditions.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A method of forming a carbon fiber structure comprising:electrospinning a polymer solution to form a polymeric material fiberstructure; and heat treating the polymeric material fiber structure toconvert the polymeric material fiber structure to a carbon fiberstructure including catalytic material particles supported on the carbonfiber structure.
 2. The method of claim 1, wherein the polymer solutioncomprises a catalytic material precursor.
 3. The method of claim 2,wherein the catalytic material precursor is dissolved in the polymersolution.
 4. The method of claim 2, wherein the catalytic materialprecursor comprises solid catalytic material particles dispersed in thepolymer solution.
 5. The method of claim 1, wherein the catalyticmaterial particles are nucleated on surfaces of carbon fiber of thestructure during heat treating.
 6. The method of claim 1, wherein thecatalytic material particles have an average particle size of less thanabout 50 nanometers.
 7. The method of claim 1, further comprisingforming the polymer solution by dissolving a polymeric material and asalt of a catalytic metal.
 8. The method of claim 7, further comprisingmixing the polymer solution to form a homogenous polymer solution priorto electrospinning.
 9. The method of claim 1, wherein heat treatingcomprises heating the polymeric material fiber structure to atemperature within a first temperature range followed by a temperaturewithin a second temperature range greater than the first range.
 10. Themethod of claim 9, wherein the first temperature range is between about100° C. and about 500° C. and the second temperature range is betweenabout 750° C. and about 1150° C.
 11. The method of claim 1, wherein thecarbon fiber structure comprises carbon fiber having an average diameterof between about 100 nm and about 500 nm.
 12. The method of claim 1,further comprising forming an electrochemical device comprising thecarbon fiber structure as an electrode.
 13. The method of claim 1,wherein carbon fiber of the structure has a resistivity of less thanabout 0.1 Ohms-m.
 14. The method of claim 1, wherein the carbon fiberstructure is a membrane.
 15. A method of forming a carbon fiberstructure comprising: converting a polymeric material fiber structure toa carbon fiber structure; and associating a catalytic material with thecarbon fiber structure.
 16. The method of claim 15, further comprisingelectrospinning a polymer solution to form the polymeric material fiberstructure.
 17. The method of claim 15, wherein converting the polymericmaterial fiber structure comprises heat treating the polymeric materialfiber structure to form a carbon fiber structure.
 18. The method ofclaim 17, wherein heat treating comprises heating the polymeric materialfiber structure to a temperature between about 100° C. and about 500° C.followed by a temperature between about 750° C. and about 1150° C. 19.The method of claim 15, wherein associating the catalytic material withthe carbon fiber structure comprises forming catalytic materialparticles supported on the carbon fiber structure.
 20. The method ofclaim 19, comprising nucleating the catalytic material particles on thecarbon fiber structure during heat treatment.
 21. The method of claim19, wherein the catalytic material particles have an average particlesize of less than about 50 nanometers.
 22. The method of claim 15,wherein the carbon fiber structure comprises carbon fiber having anaverage diameter of between about 100 nm and about 500 nm.
 23. Themethod of claim 15, further comprising forming an electrochemical devicecomprising the carbon fiber structure as an electrode.
 24. A structurecomprising: a carbon fiber structure including at least one carbon fiberhaving a length of greater than about 500 microns; and a catalyticmaterial supported on the carbon fiber structure.
 25. The structure ofclaim 24, comprising catalytic material particles supported on thecarbon fiber structure.
 26. The structure of claim 25, wherein thecatalytic material particles have an average particle size of less thanabout 50 nanometers.
 27. The structure of claim 25, wherein thecatalytic material particles are formed on a surface of a carbon fiberof the structure.
 28. The structure of claim 25, wherein the catalyticmaterial particles are dispersed substantially throughout the carbonfiber structure.
 29. The structure of claim 24, wherein the catalyticmaterial concentration is substantially constant throughout the carbonfiber structure.
 30. The structure of claim 24, wherein the catalyticmaterial concentration is less than 0.2 mg catalytic material/cm² areaof the carbon fiber structure.
 31. The structure of claim 24, whereincarbon fiber of the structure has a conductivity of less than 0.1 Ohm-m.32. The structure of claim 24, wherein the catalytic material is ametal.
 33. The structure of claim 24, wherein the carbon fiber structureincludes at least one carbon fiber having a length of greater than about1 cm.
 34. The structure of claim 24, wherein carbon fibers of the carbonfiber structure are solid.
 35. The structure of claim 24, wherein amajority of the carbon fibers of the structure have a length of greaterthan about 500 microns.
 36. The structure of claim 24, wherein carbonfiber of the structure has an average diameter of less than about 500nanometers.
 37. The structure of claim 24, wherein the structure is anelectrode.
 38. The structure of claim 24, wherein the carbon fiberstructure has a thickness of less than 10 micron.
 39. The structure ofclaim 24, wherein the carbon fiber structure is a membrane.
 40. Anelectrode comprising: a carbon fiber structure; and a catalytic materialsupported on the carbon fiber structure, wherein the catalytic materialconcentration is less than 0.2 mg catalytic material/cm² area of thecarbon fiber structure.
 41. The electrode of claim 40, comprisingcatalytic material particles supported on the carbon fiber structure.42. The electrode of claim 40, wherein the catalytic material particleshave an average particle size of less than about 50 nanometers.
 43. Theelectrode of claim 40, wherein the catalytic material particles areformed on a surface of a carbon fiber of the structure.
 44. Theelectrode of claim 40, wherein the catalytic material concentration issubstantially constant throughout the carbon fiber structure.
 45. Theelectrode of claim 40, wherein the catalytic material concentration isless than 0.1 mg catalytic material/cm² area of the carbon fiberstructure.
 46. The electrode of claim 40, wherein the catalytic materialconcentration is between 0.02 mg catalytic material/cm² area of thecarbon fiber structure and 0.2 mg catalytic material/cm² area of thecarbon fiber structure.
 47. The electrode of claim 40, wherein thecatalytic material is a metal.
 48. The electrode of claim 40, whereinthe electrode has a thickness of less than 1 micron.